This invention relates to a tuning system for a terminal in a CATV system.
CATV systems have been so arranged in the past that TV signals in a number of channels are transmitted downstream from a CATV center through a cable, and at each terminal of the same system, an electronic tuning converter converts the channel of the TV signal that has been selected into a specific channel frequency which is adapted to be received by a TV receiver. The electronic tuning converter is of a construction wherein a tuning voltage is supplied from a channel selecting device, a selected channel is determined by a signal from a local oscillator, the local oscillation frequency of which is determined by the tuning voltage, and the selected channel is further converted to the specific channel frequency.
In the above described conventional arrangement, however, drifts caused in the elements or in the tuning voltage tend to create a deviation in the tuning from the desired value.
In the CATV system, various programs, some charged or pay programs and some not charged, are supplied. Hence the tuning deviation becomes a problem. That is, when a tuning deviation occurs to an extent that not the selected channel but the next adjacent channel is received, the possiblilty exists of receiving the next channel despite an indication of reception of the selected channel. Then, if the selected channel is a non-pay channel and the next adjacent channel is a pay channel, the subscriber may receive the pay program and not be charged for it.
Furthermore, if a tuning voltage is received from outside the system, any desired channel can be received regardless of the reported selected channel. In this way, reception of a pay program while reporting to the center the reception of a a no-charge program is made possible.
An object of the present invention is to provide a tuning system which can prevent erroneous reception due to accompanying drift.
An embodiment of the present invention will now be described in detail with reference to the accompanying drawing.